It is common in the field of orthopedic surgery to perform operations involving bones where the procedural demands require close approximation of one bone to another. For example, procedures in which a joint is removed and two bones are surgically fused together requires first the apposition of the bones and then continued apposition throughout the phases of bone healing. For this purpose, a variety of devices are available to the surgeon such as pins, wires, screws, plates and staples. The surgeon approaches the condition with this variety in mind and generally will choose the device with the best profile for completing the task. For example, when strict axial compression of a long bone fracture is desired, a bone screw is typically selected. For very small bone fragments or avulsion type injuries, wires and pins are typically selected. And when the angle of approach to approximation is too obtuse for either screws or wires, such as with two flat bones ends, a staple is often selected to maintain bone alignment.
A variety of bone staples have been developed for use in various surgical procedures. Bone staples are commonly configured as a U-shaped member, having two legs and a central web or crown. Bone staples are available in variations of size, material, and insertional tools. For size variations, larger format staples are used for larger bones, and likewise smaller staple sizes are used for smaller bones. As staple width or height dimension increases or decreases respectively the leg and crown girth may be increased or decreased relative to the stability of the staple when implanted (e.g. larger girth crown for larger staples). Staples may also be made from different materials, including stainless steel, titanium, plastics, and shape-memory alloys such as Nitinol. Insertional tools vary among staple manufacturers but generally are specialized to implant or explant a particular design.
Some staples are delivered to the surgeon in a “pre-bent” configuration, for example, when the staples are inserted while bent and upon removal of the insertion tool revert to a pre-bent shape. Variations of staples also include modifications such as grooves on the legs, spikes on the crown, step changes on the crown, and grooves or hooks for insertion tools. Grooves are placed on the legs on their interior or exterior in order to enhance resistance to the staple pulling out of the bone tissue, which would be counter to the desired effect of the staple. This problem is fairly common and one or both loose legs can lead to failure of an implant. Spikes on the undersurface of the crown assist with maintenance of the staple against the substrate and stability of the overall construct. Step changes on the crown can occur co-axially with the legs or in the horizontal plane. Co-axial step changes in the crown account for particular bone procedures where one bone may sit higher or lower than another while they are opposed. This allows the staple to seat flush with the bone surfaces underneath. Step changes on the horizontal plane have been developed to aid in the compressive effort of insertional tools when they are used to ‘pull’ the legs of the staple together.
Compression of bone ends through the healing process is beneficial in some applications. For some applications, staples do not provide compression and serve as splintage of bone ends. In other applications, static compression is provided when one leg of a staple is pulled towards the other, either due to a shape memory effect of certain metals, the geometry change of prying the crown apart with certain tools, or by exploiting a pre-bent effect that some instrumentation can maintain until the staple is inserted. Regardless of the methods demonstrated in the current state of the art, the compression achieved at the time of insertion is static and does not increase over time or with deleterious distractive forces at bone ends. In addition, current compressive staples are designed to create axial compression, where one leg compresses towards the other along an axis created by the crown. Because demands of surgical operations often vary, axial compression may not be desired based on the position of the staple when it is inserted. For example, oblique positioning of a compression staple across a linear fusion site will introduce shear, a deleterious force when attempting to compress bone ends symmetrically.
There are several principles followed for insertion of a bone staple that are common to all of the variants. First the size must be appropriate. Too small a staple, and the legs can violate the fusion site, or the device may be inadequate to resist the distractive forces on the bone ends. Second, any ‘pilot’ holes or pre drilling for the staple legs must be sufficiently small to avoid removal or displacement of bone required for the staple to seat securely in the bone. Loss of bone purchase can lead to loosening or inability to use this fixation method. Third, the staple legs should typically sit directly perpendicular to the fusion or fracture line since a deleterious shear force will be introduced if the implant is placed obliquely. This principle is particularly important when using a compressive staple. Fourth, the staple should have some obliquity to insertion of the legs, since discrete or strict axial applications are less effective at resistance to pulling out of the bone.
It has now been recognized that needs exist for improved bone staples, improved methods of use of bone staples, and improved staple implantation devices and procedures. It is to the provision of improved bone staples, methods and implantation devices and procedures meeting these and other needs that the present invention is primarily directed.